Decanter centrifuges comprise a drum rotatable in a housing comprising a lower part and a lid, which is normally cylindrically dome-shaped. The lid may be hinged to the lower part along a side thereof.
Alternatively to hinging the lid to the lower part, the lid may be provided as a free part to be lifted from the lower part for opening the housing, possibly using a crane if the lid is heavy.
It should be noted that lids of decanter centrifuges may weigh more than 60 kg, e.g. 300 kg or even more, depending on the size of the centrifuge.
For decanter centrifuges with a hinged lid it is known to provide springs for assisting the raising of the lid during opening.
It should be noted that the lid of a decanter centrifuge is normally opened once a week or once every second week for cleaning and/or maintenance. The duration of the intervals between opening the lid depends however on the use of a specific centrifuge. Thus some centrifuges may be opened daily.
A decanter centrifuge of a generally known construction is shown schematically in FIGS. 1 and 2, and a known hinge with an opening assisting one-way spring is schematically shown in FIG. 3. The Figures are explained below.
A hinge with an opening assisting spring as shown in FIG. 3 has proven to be a good solution for lids up a certain size. However it has not proven possible to provide a satisfactory solution on the basis of the construction shown in FIG. 3 for a lid with a weight of 300 kg.
One object of the present invention is to provide a centrifuge with a hinge with an opening assisting spring unit that will provide a satisfactory solution for heavy lids, that can, but are not limited to weigh 300 kg or more.
According to the present invention a connection member is connected to the spring unit to stress it during at least a final part of the lid's rotation around the hinge axis to the fully opened position, whereby the spring unit will urge the lid through the connection member in a direction from the fully opened position towards the closed position, when the lid is in the fully opened position, and that the lid has a position of stable equilibrium in an intermediate position of the lid between the closed position and an open, sometimes substantially vertical position, in which the centre of gravity is positioned substantially vertically above the hinge axis. Providing for the spring unit to urge the lid from the fully opened position towards the closed position provides for the spring unit to also assist closing the lid from the fully opened position, in which the lid tends to stay open due to the fact that the centre of gravity of the lid in this open position is located on the distal side of the vertical plane through the hinge axis, and it provides for the spring unit to catch the lid as its centre of gravity passes the vertical plane through the hinge axis and the lid tends to fall backwards. Further the position of stable equilibrium provides for the lid to stay in a position in which the centre of gravity has passed from the distal side to the proximal side of the vertical plane through the hinge axis without having to fall to the closed position, which provides for safer and more controlled operation by an operator.
In one embodiment the centrifuge is a decanter centrifuge and the drum has a horizontal axis of rotation.
Preferably, the lower part comprises a sealing surface and the lid comprises a sealing surface extending along a rim of the lid, the two sealing surfaces being in engagement when the lid is in the closed position, and preferably a number of fasteners are provided along the rim of the lid for securing the lid in the closed position and for pressing the sealing surfaces against each other.
Preferably, at least a part of the connection member connected to the hinge part is shiftable between limits relative to a force transferring interface between the connection member and the spring unit when the spring unit is in a position, in which it has its neutral extension. This provides for a dead band interval within which the lid will rotate without being urged by the spring unit, which facilitates construction and dimensioning of the spring unit and the elements mounting it.
In a preferred embodiment, a vertical plane parallel to the hinge axis and passing through the hinge point is, when the lid is in its closed position, located between the centre of gravity of the lid and the hinge axis, and the hinge allows limited vertical movement of the lid in the closed position. This embodiment is particularly advantageous when a gasket on the lid provides one of the sealing surfaces. In this embodiment, the gasket is not compressed by the lid rotating from an open to the closed position, but is only compressed when the fasteners are secured. Thus, an effect of the pressure of the gasket opening the lid the first 5-10° from the closed position is avoided.
In a further embodiment the spring unit is a mechanical spring unit, the connection member extends beyond a longitudinal extension of the spring unit, the spring unit is confined at its longitudinal ends between first abutments, which are fixed relative to the second of the hinge parts, and the connection member comprises second abutments for compressing the spring unit from either longitudinal end thereof. Accordingly, the spring unit will be compressed when the lid is rotated away in either direction from a neutral position e.g. at the vertical position mentioned above.
In a preferred embodiment, the mutual distances between the first abutments and the second abutments, respectively, are different. This feature provides for a dead band interval as mentioned above.
In a further embodiment, the spring unit is hollow, and the connection member extends through the spring unit.
In a preferred embodiment, the spring unit comprises a stack of Belleville washers. Using Belleville washers provides for more easily obtaining the desired characteristic of the spring unit.
In a further embodiment, a washer element is provided at one of the longitudinal ends of the spring unit, the spring unit abutting the washer element, the connection member is passing through the washer element, the connection member being longitudinally slidable relative to the washer element, the second abutment is engageable with the washer element, and a stop is provided restricting movement of the washer element in a longitudinal direction away from the spring unit, the stop thus providing a first abutment.
Preferably, a washer element is provided at either longitudinal end of the spring unit. This provides for the above mentioned effect of compressing the spring unit when the lid is rotated in either direction from a neutral position.
In a preferred embodiment, the washer element at the longitudinal end of the spring unit opposite the hinge point is movable in the longitudinal direction between two stops. Thus a stop is provided for the opening movement of the lid establishing the fully opened position.
In an alternative embodiment the connection member is connected to a piston of a pneumatic or hydraulic cylinder assembly providing the spring unit.